Based on comments on this forum, I believe your tower entry time will be 15 minutes after the entry time for the church. There seems to be no flexibility there. Perhaps that is made clear during the online booking process. I am not familiar with that because I bought my ticket in person at the Tourist Office.
I haven't ascended one of the towers, so I really don't know how long that takes. I believe I spent about 90 minutes in the church, including the small museum. I used the audio guide but am not at all religious, so others might well spend more time. If you hope everyone will finish up at the church at about the same time, you'll need to buy earlier tickets for the tower-climbers than for the rest of your party. I hope someone on the forum can give you a time estimate. If not, try Googling something like time for La Sagrada Familia tower. It's amazing what Google can turn up; any number will of course just be an estimate.
Important: There is a shop with some attractive merchandise. Once you enter the shop, you cannot return to the church. Be sure everyone in your group knows that. That's not where you want the non-tower people to go to kill time at the beginning of their visit!
I chose a first-entry time-slot and was very pleased with that decision because the church was nearly empty when I walked in. For those who choose the tower option, I don't know that there's so much advantage to choosing the first time-slot from the crowding standpoint since additional people will be admitted to the church during their tower visit.
My visit was in August, and I thought the windows were beautiful early in the morning, but I've only seen them at that time of day so I can't say whether some other time would be appreciably better. However, this is one of the Barcelona sights where you must buy a timed ticket and be on time. That's a lot easier to do if you're not planning to go somewhere else first. Up to this point, I have seen no reports that Sant Pau has that limitation. I believe it's a good site to plan for later in the day, because you can probably just show up when it's convenient for you. I spent two hours at Sant Pau and really had to rush to finish before it closed. I think an additional room has been opened since then.